Responsibility for the bombing was claimed by al-Qaeda in the Islamic State of Iraq and the Levant (ISIL), whose forces are fighting on the rebel side against President Assad.

The bombing is believed to be the first time that ISIL, a joint Iraqi-Syrian group, has struck directly against the League of the Righteous. Retaliation by the League, which has several thousand fighters at its command, could drag Iraq back into its own sectarian civil war, which claimed the lives of 30,000 people between 2006 and 2008.

A senior Sunni politician in the southern city of Basra, Abdul-Kareem al-Dussary, was shot and killed Friday night in what appeared to be an initial revenge attack for the Baghdad bombings.

The bombing took place at a rally organised to introduce the League's candidates for Iraq's parliamentary elections, which are to be held amid a massive security operation on Wednesday.

As men and women in colourful Arab medieval costumes did a short performance of a 7th century Shia religious play, the rally was hit by two huge explosions about ten minutes.

Witnesses spoke of pandemonium in the aftermath, with security forces posted to guard the rally firing automatic weapons in all directions.

"I saw four charred bodies and several wounded people asking for help," said one man. "There were also several damaged cars. Then, other blasts took place. People were in panic."

The rally was addressed by Asaib Ahl al-Haq's leader, Sheik Qais al-Khazali, a young cleric who spent years in US custody but was released in a part-prisoner swap for one of the five kidnapped British men, Peter Moore. His four bodyguards were killed while in captivity.

After the first explosion, Mr Khazali's security entourage surrounded him and then rushed him to an armored SUV.

At Friday's rally, Mr Khazai challenged Sunni militants holding parts of two cities in Iraq's western Anbar province, which is predominantly Sunni.

"We are ready and prepared to defend this nation," he said. "Let it be known that Asaib will be the remedy."

A website used by militants said that the bombings were to avenge what it called the killing of Sunnis and their forced removal from their homes by Shiite militias.

The League of the Righteous, known in Arabic as Asaib Ahl al-Haq, was formed around 2006 as a splinter group from the Mehdi Army militia run by the radical Shia cleric Moqtada al-Sadr.

Trained by Iran's Revolutionary Guards, it styles itself as the defender of Iraq's Shia communities against violence from Sunni extremists loyal to al-Qaeda. Western diplomats, though, say it formed Shia death squads that carried out tit-for-tat killings against both ordinary Sunnis and al-Qaeda militants during the 2006-8 civil war.

Al-Qaeda's recent resurgence in Iraq has sparked concerns that the League, which has now officially embraced peaceful politics, could activate its death squads once again. Friday's bombing could prove to be just that kind of trigger.

Last year, the death toll in Iraq climbed to its highest levels since the civil war. The United Nations said that 8,868 people were killed in 2013. More than 1,400 people have died in the first two months of this year alone.